A conventional contact sensing display structure has a contact sensing element under a display. The contact sensing element is placed at a protected space. When the display is touched, its force is applied to a contact sensing film through the display. For example, an electro (electric) mechanical film (EMF) is preferably used as the contact sensing element in order to detect the contact, for example. A LCD display is used as the display, for example.
A more preferable method of measuring a change of charges in the EMF is to use a charge amplifier. In the charge amplifier, current between electrodes of the film is measured in a short circuit. An operation amplifier, resistors, and condensers constitute the charge amplifier connection. The resistors and the condensers are used for balancing the input impedances of the operation amplifier and they do not affect the transfer function of the connection. The resistor is used for limiting D.C. amplification (for example, see Patent document 1).
However, in the contact sensing display structure, when the user continues to push the display and leakage current flows into the contact sensing element, an output of the contact sensing element fluctuates. Accordingly, it may be impossible to accurately detect pressing by a manipulation input on the display.
[Related-Art Documents]